As a common component in analog circuits, current mirrors are widely applied in the biasing stage or the amplifying stage. Therefore, the stability and the accuracy of the output current are crucial to the quality of a current mirror.
A prior art current mirror device is normally implemented using metal-oxide-semiconductor field effect transistors (MOSFETs). The effective channel length of a MOSFET remains constant when operating in the linear region. As the drain voltage increases, the MOSFET enters the saturation region. Ideally, the operational current of the MOSFET in the saturation is only related to the gate-source voltage (VGS) of the MOSFET. However in reality, the increased drain voltage shortens the effective channel length of the MOSFET, also known as channel length modulation effect which makes the operational current of the MOSFET also related to the drain-source voltage (VDS) of the MOSFET. Therefore, the prior art current mirror device is easily influenced by process and bias variations, thus unable to accurately provide the output current.